I'm running into the same issue - unless I find a better way to do it, I'm just going to use a standard Y-Cable on each of the rear outputs to split them (giving me 2 for the rears and 2 for the subs(which I'll use the amp to bridge)). The voltage will be the same on any of the split outputs, but the current will be split between them. I'm not sure what kind of an effect this will have - I don't believe it will cause any problems, but I'm gonna find someone 'in the know' to ask beforehand.

Hope this helps.